The invention relates to a valve body produced by powder metallurgical methods that exhibits high thermal and wear resistance and to a valve for combustion engines fitted with said valve body.
Inlet and outlet valves for combustion engines must satisfy high requirements with respect to thermal endurance and wear resistance. Especially for the high-compression engines of multi-valve and electronic control design it has been more and more difficult to find materials that on a long-term basis meet the needs linked with the high temperatures arising at the outlet. Valves manufacture has become more and more sophisticated and expensive and the material and machining costs involved in the process reflect this.
For the production of valve bodies or entire valves powder metallurgical methods have been proposed occasionally. Such powder-metallurgical processes have been used in the manufacture of valve seat inserts but have not yet gained widespread acceptance in the production of valve bodies or valves proper. Reasons for this were an insufficient durability of the materials and their inadequate performance in terms of thermal resistance.
To enhance the performance of conventionally manufactured valves inductive hardening or armoring methods have been employed to process particularly stressed areasxe2x80x94particularly for the valve seat. This measure aims at keeping wear within acceptable limits with consideration to be given to the fact that even with this technology valve temperatures of between 800xc2x0 C. and 900xc2x0 C. should not be exceeded. Nevertheless, it is increasingly difficult to meet this requirement with present-day engines.
Producing valves and valve bodies by conventional processes has become extremely complex, particularly with respect to seat armoring. Starting out with a rod segment the valve body is produced initially by heating, upsetting, calibrating and turning, to which then a rod segment is attached by friction welding. Additional working steps comprise straightening, turning, grinding, and build-up welding, grinding and heat treatment to produce the finished valve with seat armoring. Especially, the seat armoring step involving build-up welding may lead to faults associated with unacceptably high reject rates.
Solutions aimed at providing a suitable seat armoring in the form of powder metallurgically produced armorings were sought but could not reach a mass production status. Applying the seat armoring did not bring about a reduction of the failure rate. Such an armoring produced by powder metallurgical methods rather turned out to be susceptible to crack formation during subsequent processing steps.
With respect to the materials and additional processing steps required to produce armored or hardened seats it appears to be desirable to make valve bodies of a homogeneous material in as few steps as possible with the material having the necessary wear resistance, service life and heat dissipation characteristics and with the body being connected to a rod thus forming a valve.
Methods for the production of at least the wear lining of high-duty sintered parts in connection with the control of valves in an internal combustion engine are known from DE 41 04 909 A1. The sintered parts thus produced by powder metallurgical steps feature a high chromium and carbon content and are employed as cams for valve control purposes. Using such sintered parts for valve bodies has not been envisaged.
For the production of sophisticated shaped parts powder metallurgy frequently offers advantages over conventional techniques in that the material characteristics can be optimized and the number of processing steps reduced.
Therefore, it is the object of the invention to produce valve bodies for valves by powder metallurgical methods from a material suitable for the purpose, particularly with a view to reducing the manufacturing expenditure. This method shall render a seat armoring dispensable and the valve, respectively the valve body shall feature thermal conductivity properties sufficient for temperature control. The valve body shall be connected with a conventionally manufactured valve rod by butt-joining methods thus forming a functioning and durable valve.
This object is achieved by providing a powder metallurgically manufactured valve body having the following composition by weight:
0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to 5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V;  less than 2% other elements, the remainder being Fe.